Low-dose methortrexate therapy suppresses autoimmune arthritis in human and animal models. It is our hypothesis that the effect of methotrexate in the treatment of rheumatoid arthritis is due to the inhibition of aminoimidazole-carboxamide ribotide transformylase, a folate-dependent enzyme which catalyzes the last step in the de novo biosynthesis of inosine monophosphate. The resulting accumulation of aminoimidazole carboxamide riboside inhibits adenosine deaminase, therefore, interfering with normal adenosine metabolism. It is well known that children with adenosine deaminase deficiency have severe-combined-immunodeficiency syndrome. This suggests that adenosine deaminase activity is key to immune competence and is associated with the mechanism of efficacy in methotrexate therapy of rheumatoid arthritis. Several studies indicate that supplemental folinic acid (5- formytetrahydrofolate) used in large doses during low-dose methortrexate therapy for rheumatoid arthritis causes a flare in joint inflammation. However, supplemental folic acid (pteroylglutamic acid) does not lessen the efficacy of the therapy. We further hypothesize that if methotrexate efficacy is driven by aminoimidazole carboxamide ribotide transformylase inhibition, folic acid supplementation should not alter urinary levels of aminoimidazole carboxamide, adenosine, and deoxyadenosine, while folinic acid supplementation should prevent the accumulation of these compounds. Our hypotheses will be tested both in patients with rheumatoid arthritis and in Lewis rat adjuvant arthritis. Objectives include A) to determine whether supplemental folic acid and folinic acid during methotrexate therapy normalize adenosine metabolism in patients with rheumatoid arthritis. The information obtained from the proposed research will enhance the understanding of the biochemical action of antifolates/ antimetabolites that are effective in the treatment of human and animal arthritis. To date, six patients have been enrolled in the trial. Because the trial is blinded, no data is yet available. It is planned inthe upcoming year to enroll patients, up to a total of 50 patients.